Nonequilibrium axial charge production in expanding glasma flux tubes

Abstract

Axial charge production at the early stage of heavy-ion collisions is investigated within the framework of real-time lattice simulations at leading order in QCD coupling. Starting from color glass condensate initial conditions, the time evolution of quantum quark fields under classical color gauge fields is computed on a lattice in longitudinally expanding geometry.We consider simple color charge distributions in Lorentz contracted nuclei that realize flux tube-like configurations of color fields carrying nonzero topological charge after a collision.By employing the Wilson fermion extended to the longitudinally expanding geometry, we demonstrate the realization of the axial anomaly on the real-time lattice. I. INTRODUCTIONIn relativistic heavy-ion collisions, CP-violating configurations of color gauge fields can be generated locally either by gauge field dynamics at the instant of a collision or sphaleron transitions at later times [1][2][3][4][5]. Quarks interacting with such gauge fields induce the imbalance of axial charge due to the quantum phenomenon of axial anomaly. In presence of a strong U(1) magnetic field, which may be generated in off-central collisions, the axial charge asymmetry can be converted to a flow of electric current along the magnetic field [6]. This phenomenon is called chiral magnetic effect (CME) [7][8][9]. Experimental searches for this novel phenomenon have been carried out at RHIC and the LHC [10-12], where a charge dependence of azimuthal correlations was measured [13]. However, the observation of the CME in heavy-ion collisions still remains inconclusive due to large backgrounds [14].On the theory side, there have been numerous developments in the description of the transport phenomena associated with the CME based on the chiral kinetic theories [15][16][17][18][19][20] and the anomalous hydrodynamics [21][22][23][24][25][26]. To make predictions of observable consequences of the CME, some of these frameworks need the information of the axial charge distribution as an initial condition as well as * ntanji@ectstar.eu arXiv:1805.00775v2 [hep-ph] 19 Jul 2018 the space-time distribution of the magnetic field. Since the lifetime of the magnetic field is expected to be short < ∼ 1 fm/c [27][28][29], the understanding of the axial charge production at the early stage of heavy-ion collisions is indispensable in order to make reliable predictions about the CME.At high energies, colliding heavy-ions can be described in terms of the effective theory of color glass condensate (CGC) [30][31][32]. By a collision, strong color electromagnetic fields are generated, and the system expands to the longitudinal direction in a nearly boost-invariant way. Even though the QCD coupling is weak g 1, this system, called glasma [2], is strongly correlated because the gauge fields are inversely proportional to the coupling constant A ∼ 1/g as a consequence of the gluon saturation. Nonperturbative dynamics of these gauge fields can be computed by classical(statistical) gauge field simulations on the real-time ...

Fluctuations of topological charge and chiral density in the early stage of high energy nuclear collisions

“…These domains remain microscopic with respect to the nucleon, in the sense that in physical units the correlation length sits in between the 10% and the 20% of the proton radius. The correlators and correlation lengths that we have found agree with previous studies [21,30,32,43,44].…”

“…We have analyzed the case of a longitudinal boost invariant expansion that is more relevant for the modeling of the early stage of the collisions. Despite some amount of information available in the literature on the formation of topological charge domains in the early stage of high energy nuclear collisions, see for example [14,30,32], a concrete calculation that takes into account the full evolution of the gluon field with the initial condition given by the MV model is still missing: we aim to cover this lack of information here. Besides its theoretical own interest, this study is relevant for the phenomenology of high energy nuclear collisions: for example, just recently it has been pointed out that the production of chiral density in the early stage can affect the polarization of the Λ particles [31,33] and puts constraints on the strength of the magnetic field produced in the collisions: it is therefore important to have both a qualitative and a quantitative understanding of the n 5 that is produced in the early stage where classical gluon fields dominate the dynamics.…”

“…Since ρ T is charge conjugation and parity (CP) odd, it might be the source of large CP violating fluctuations in heavy ion collisions, see Refs. [26][27][28][29], and [30][31][32][33][34][35] for recent reviews.…”

QCD thermalization: Ab initio approaches and interdisciplinary connections

“…In addition, transient anomalous charge production in strong-field QCD was studied in Refs. [358,359].…”

Gauge ambiguity of the quark spectrum in the Color Glass Condensate

“…In order to address these questions with a setup that allows an analytical treatment, we consider an SU (2) gauge group with a very special configuration of color sources for the two projectiles, which was introduced in [26]. Let us first recall the link between these sources and the color field generated immediately after the collision.…”